NAME

LIBRARY

SYNOPSIS

DESCRIPTION

The mlock() system call locks into memory the physical pages associated
with the virtual address range starting at addr for len bytes. The
munlock() system call unlocks pages previously locked by one or more
mlock() calls. For both, the addr argument should be aligned to a
multiple of the page size. If the len argument is not a multiple of the
page size, it will be rounded up to be so. The entire range must be
allocated.
After an mlock() system call, the indicated pages will cause neither a
non-resident page nor address-translation fault until they are unlocked.
They may still cause protection-violation faults or TLB-miss faults on
architectures with software-managed TLBs. The physical pages remain in
memory until all locked mappings for the pages are removed. Multiple
processes may have the same physical pages locked via their own virtual
address mappings. A single process may likewise have pages multiply-
locked via different virtual mappings of the same pages or via nested
mlock() calls on the same address range. Unlocking is performed
explicitly by munlock() or implicitly by a call to munmap() which
deallocates the unmapped address range. Locked mappings are not
inherited by the child process after a fork(2).
Since physical memory is a potentially scarce resource, processes are
limited in how much they can lock down. A single process can mlock() the
minimum of a system-wide ‘‘wired pages’’ limit and the per-process
RLIMIT_MEMLOCK resource limit.
These calls are only available to the super-user.

RETURNVALUES

Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
If the call succeeds, all pages in the range become locked (unlocked);
otherwise the locked status of all pages in the range remains unchanged.

ERRORS

The mlock() system call will fail if:
[EPERM] The caller is not the super-user.
[EINVAL] The address given is not page aligned or the length is
negative.
[EAGAIN] Locking the indicated range would exceed either the
system or per-process limit for locked memory.
[ENOMEM] Some portion of the indicated address range is not
allocated. There was an error faulting/mapping a
page.
The munlock() system call will fail if:
[EPERM] The caller is not the super-user.
[EINVAL] The address given is not page aligned or the length is
negative.
[ENOMEM] Some portion of the indicated address range is not
allocated.

SEEALSO

HISTORY

The mlock() and munlock() system calls first appeared in 4.4BSD.

BUGS

Allocating too much wired memory can lead to a memory-allocation deadlock
which requires a reboot to recover from.
The per-process resource limit is a limit on the amount of virtual memory
locked, while the system-wide limit is for the number of locked physical
pages. Hence a process with two distinct locked mappings of the same
physical page counts as 2 pages against the per-process limit and as only
a single page in the system limit.
The per-process resource limit is not currently supported.